Adhesive

ABSTRACT

An adhesive comprising: (a) 80 to 100% of a melt blend of 40 to 98% polyalphaolelin which has been grafted with 0 to 5% of at least 1 ethylenically unsaturated carboxylic acid or derivative thereof, (b) 2 to 60% of at least one copolymer of ethylene and vinyl acetate; (c) 0 to 20% of an opacifier; (d) optional further ingredients; wherein the total amount of components (a) and (b) is 80-100% of the composition and wherein the percentages of the components are selected to total 100%.

[0001] This invention relates to an adhesive particularly but notexclusively adapted for use in engineering applications and comprisingmodified polypropylene as a base material.

[0002] Our earlier UK patent application 00265507.4 discloses a lowmelting point, low viscosity polypropylene containing adhesive suitablefor use in high-speed manufacture of packaging.

[0003] WO95/24449 discloses an adhesive composition comprising maleicanhydride-grafted propylene and grafted ethylene/vinyl acetatecopolymer.

[0004] According to a first aspect of the present invention an adhesivecomposition comprises:

[0005] (a) 80 to 100% of a melt blend of 40 to 98% polyalphaolefin whichhas been grafted with 0 to 5% of at least one ethylenically unsaturatedcarboxylic acid or derivative thereof,

[0006] (b) 2 to 60% of at least one copolymer of ethylene and vinylacetate;

[0007] (c) 0 to 20% of an opacifier;

[0008] (d) optional further ingredients;

[0009] wherein the total amount of components (a) and (b) is 80-100% ofthe composition and wherein the percentages of the components areselected to total 100%.

[0010] Percentages and other proportions referred to in thisspecification are by weight unless indicated otherwise.

[0011] An adhesive composition in accordance with the present inventionmay be applied by a doctor or other means onto a substrate. It may alsobe formed into a film which can be applied onto a substrate in a varietyof ways such as compaction. A film of the present invention can beemployed as a free-standing film. Alternatively, the adhesivecomposition may be formed into rods, pellets or otherwise shapedportions to facilitate application to a substrate.

[0012] Films or other structures formed from compositions of thisinvention may have the advantage of being printable. The films orstructures may be transparent, that is having no apparent phaseseparation. The transparency is advantageous and unexpected,particularly because ethylene vinyl acetate compositions are commonlyopaque.

[0013] The polyalphaolefin is preferably a polymer of an alphaolefin offormula (CH₂.CHR)_(n), wherein R is C₁-C₆ alkyl. The preferredpolyalphaolefin is selected from polypropylene and copolymers thereof.

[0014] The rheological behaviour of the formulations described in thisinvention is important for film forming, doctoring and other processingmethodologies for which it is suitable. The rheological characteristicsof the formulations are also important in relation to their capacity towet substrates. Typical shear and elongational viscosities of apreferred formulation are shown in FIG. 1 and FIG. 2, respectively.

[0015] The functionalised polypropylene may be produced directly byreactive modification by using radical chemistry where the functionalgroup can be isocyanate, anhydride, amine, alcohol or acid, preferablyhaving a melt flow index of 10 to 120 preferably 10 to 95, morepreferably 50 to 60 at 190° C. The functional polypropylene may also bemade by copolymerisation of propylene with suitable diolefine monomerand then subsequent modification of the pendant unsaturation. Maleicanhydride grafted polypropylenes may be employed. Alternatively2,2′-dimethyl-1,3-isopropenyl benzyl isocyanate (also known as dimethylmeta-isopropenyl benzyl isocyanate) (TMI) modified polypropylenes asdisclosed in WO96/34024, WO96/3403 1 and WO98/13398 may be employed.

[0016] The unmodified ethylene/vinyl acetate copolymer is preferably anunmodified random or statistical copolymer having a melt flow index ofnot more than 800, preferably not more than 600, more preferably notmore than 400 at 190° C. A vinyl acetate content of up to 40% in thecopolymer may be employed, preferably around 28%.

[0017] Preferred adhesives in accordance with this invention do notincorporate an opacifier. However, opacifiers which can be used includeinorganic particulates, for example, mica, talc and inorganic sulphates,carbonates, halides and pigments, Organic opacifiers include insolubleparticulate materials including polymers such as polystyrene,polyesters, polyamides, polyacrylates, polymethacrylates, inks, pigmentsand colorants.

[0018] Adhesives in accordance with this invention may be applied insolid form to a workpiece, for example, as a sheet of film, by sprayingor as pellets and then heated to cause melting of the adhesive.Conventional hot compaction using hot plates or a heated press may beemployed. Alternatively, a metal workpiece or substrate may be heated tocause melting of the adhesive.

[0019] In a further alternative laser heating may be used for materialswhich are transparent to laser radiation. Heating efficiency may beenhanced by using infrared absorbent materials in the composition.

[0020] The following results are illustrative of the good bonding whichcan be achieved using one method of heat application and one physicalform of the adhesive. Satisfactory bond strengths are achieved usingother heat sources for the activation of the adhesive, such as hotcompaction. The reduction in bond strengths as temperature is increasedreflects the intrinsic change in modulus of the adhesive formulation.The higher the content of modified polypropylene in the formulation thebetter the high temperature bond strength. Those skilled in the art candesign formulations to achieve optimal wetting, bond strength andprocessability for any given means of heat application to effect goodbonds.

[0021] Induction heating or dielectric heating may be used withcompositions to which metal particles, for example, iron filings, havebeen added.

[0022] The invention also promotes an article adapted to be secured toanother article includes a surface carrying a pre-coated layer of anadhesive in accordance with the present invention.

[0023] Compositions in accordance with this invention may bemanufactured by melt blending of the key components in a batch blunderor by using an extruder, for example, a twin-screw extruder. This is thepreferred method as the adhesives can be pelletised for processing bystandard injection moulding, extrusion or a range of other commonly usedpolymer processing methods. The adhesive can be readily formed intodifferent shapes e.g. gaskets for bonding shaped work pieces.

[0024] The adhesive formulation described is particularly well suited tothe bonding of polypropylene to polypropylene and the substrates can bein the form of sheet, woven fabric or other physical forms. Theadhesives in accordance with this invention may also be used for thebonding of many different substrates or combinations of substratesincluding: aluminium, steel, cellulosic materials, melamine laminatesand leather.

[0025] The invention finds a particular application in manufacture ofengineering mechanical components or articles wherein a high bondstrength is desired. Bonding of polypropylene surfaces is facilitated.

[0026] In a continuous process a film in accordance with this inventionmay be dispensed between surfaces to be bonded, nipped by compression ofthe surfaces and then heated to cause melt adhesion. Large flat surfacescan be bonded in a continuous process. Large curved or other shapedsurfaces can be thermoformed in a semi-continuous process. This is allimprovement over prior processes in which the adhesive is applied byspraying. Spraying results in the poor control of the thickness,integrity and density of the adhesive layer. Laminated panels may bemanufactured using aluminium or other metal sheets.

[0027] The invention is further described by means of example but not inany limitative sense, with reference to the accompanying drawings ofwhich:

[0028]FIG. 1 is a graph comparing shear viscosity vs. shear rate or foran adhesive in accordance with this invention.

[0029]FIG. 2 is a graph comparing elongational viscosity vs. shear ratefor an adhesive in accordance with this invention.

EXAMPLE 1

[0030] An APV 2030, 30 mm diameter screw, 40:1 L/D, co-rotatingtwin-screw extruder was used in a continuous method of manufacturing ofthe adhesive. A mixture of maleic anhydride functionalised polypropyleneand ethylene/vinyl acetate copolymer (EVA) pellets at a weight ratio of80:20 was fed continuously into the hopper of the extruder. The feedrate was approximately 17.5 kgh⁻¹ and the extrusion temperature wasapproximately 170° C. The extrudate was then passed through a waterbath. The solidified extrudate was finally cut up into suitable sizepellets for further applications.

EXAMPLE 2

[0031] Approximately 4 kg of the adhesive pellets as described inExample 1 was dried overnight in an oven at 60° C. The pre-dried pelletswere then extruded into rolls of film of approximately 250 mm in widthand 70 microns in thickness. The extrusion temperature was approximately180° C. and the wind-up speed was 1.5 mmin⁻¹. This process was laterscaled up to manufacture adhesive film of 1.1 m in width and 60 micronsin thickness.

EXAMPLE 3

[0032] The thermal behaviour of the adhesive as described in Example 1was examined by differential scanning calorimetry and melt rheometry.The onset and the peak of melting of the adhesive were found atapproximately 130° C. and 145° C. respectively. Typical shear andelougational viscosities of the preferred formulation, measured at 170°C., are shown in FIG. 1 and FIG. 2 respectively.

EXAMPLE 4

[0033] The bond strength of the adhesive as described in Example 1 wasexamined by lap-shear test of single overlap joints (with reference toBS 5350: Part C5: 1990, ASTM D 1002-94 and D3164-97) and T-peel test(with reference to BS 5350: Part C12:1994 and ASTM D 1876-95). Sheets ofglass-filled polypropylene, aluminium and mild steel sheet were used toprepare all the test samples. The adhesive films, as described inExample 2, were used to prepare the bonds. The sample surface wasdegreased with acetone prior to bonding. No further surface treatmentwas carried out. The bonding temperature and contact time were 155° C.and 1 minute respectively. A gentle pressure was applied to ensure agood contact between the adhesive and the bonding surfaces. Results areshown in Tables 1 and 2. TABLE 1 Variation of shear strength (values inMPa) of the adhesive film, as described in Example 2, on variousadherends with temperature. Temperature (° C.) −40 −20 0 18 40 70 100130 Mild steel 20.80 20.30 18.32 14.33 11.29 7.02 5.77 3.01 Aluminium12.31 12.62 12.19 11.48 8.88 7.23 2.61 2.37 Glass-filled PP — — — 11.37— — — —

[0034] TABLE 2 Variation of peel strength (values in N/mm) of theadhesive film, as described in Example 2, on various adherends withtemperature. Temperature (° C.) −40 −20 0 18 40 70 100 130 Mild steel4.18 3.74 3.92 6.36 5.78 3.67 3.37 1.57 Aluminium 1.86 1.97 2.58 4.785.67 4.13 2.60 1.38 Glass-filled PP — — — 3.70 — — — —

EXAMPLE 5

[0035] Semi-translucent isotropic polypropylene, approximately 3 mmthick, was laser-welded at different laser pass rates to 0.7 mm mildsteel plate, using an adhesive film as described in Example 2. Theeffectively bonded area varied greatly dependent on the speed of thelaser over the joint. The laser-welded samples were then cut into 25 mmparallel strips for testing their lap-shear strength. Results are shownin Table 3. The most effective laser speed over the material to effectgood bonding in this example is in the region of 200 mmmin⁻¹ but thischanges with laser power and the substrates being bonded. When thesubstrate is opaque but conducts heat rapidly, as in the case for mostmetals, it is possible to achieve similar bond strengths by applying thelaser heating to the metal surface. TABLE 3 Variation of shear strengthof the laser-welded samples with laser speed. Laser speed (mmmin⁻¹) 100150 175 200 225 250 Maximum strength (MPa) 4.2 6.8 12.1 16 10.3 9.5

EXAMPLE 6

[0036] Composites comprising a sandwich structure of hot-compacted PPsheet/PP foam or PP honeycomb/PP sheet were prepared in a continuouslaminating process using the adhesive film, as described in Example 2,as bonding agent. The laminating temperature and speed were 160° C. and3 mmin⁻¹ respectively. These composites exhibit good impact resistantproperties and recyclability.

1. An adhesive comprising: (a) 80 to 100% of a melt blend of 40 to 98% polyalphaolefin which has been grafted with 0 to 5% of at least 1 ethylenically unsaturated carboxylic acid or derivative thereof, (b) 2 to 60% of at least one copolymer of ethylene and vinyl acetate; (c) 0 to 20% of an opacifier, (d) optional further ingredients; wherein the total amount of components (a) and (b) is 80-100% of the composition and wherein the percentages of the components are selected to total 100%.
 2. An adhesive as claimed in claim 1, wherein the polyalphaolefin is selected from propylene and copolymers thereof.
 3. An adhesive as claimed in claim 1 or 2, wherein the polyalphaolefin has a melt flow index of 10-95 at 190° C.
 4. An adhesive as claimed in claim 3, wherein the melt flow index is 50-60 at 190° C.
 5. An adhesive as claimed in any preceding claim, wherein the copolymer of ethylene and vinyl acetate has a melt flow index of not more than 800, preferably not more than 400 at 190° C.
 6. An adhesive as claimed in any preceding claim, wherein the copolymer of ethylene and vinyl acetate has a vinyl acetate content of up to 40%.
 7. An adhesive as claimed in claim 6, wherein the vinyl acetate content is 28%.
 8. An adhesive composition as claimed in any preceding claim, in the form of a film.
 9. An adhesive composition as claimed in claim 8, wherein the film is a free standing film.
 10. An adhesive composition as claimed in any of claims 8 and 9 having 0% opacifier, wherein the film is transparent.
 11. A method of uniting surfaces of juxtaposed layers using an adhesive composition as claimed in any preceding claim, wherein the composition is in the form of a film, the method comprising the steps of: contacting the film with a first layer, contacting the second layer to the film, and causing the film to melt and subsequently solidify on cooling to form a united article.
 12. A method of uniting juxtaposed layers using an adhesive composition as claimed in any of claims 1 to 10, wherein the composition is in the form of strands or pellets, the method comprising the steps of: applying the adhesive to a first layer contacting the second layer to the adhesive and causing the adhesive to melt and subsequently solidify on cooling to form a united article.
 13. A method as claimed in claim 11 or 12, wherein one or more of said layers is heated, to cause melting of the adhesive.
 14. A method as claimed in any of claims 11 to 13, wherein the layers are compacted against the adhesive using hot plates or a heated press.
 15. A method as claimed in any of claims 11 to 14, wherein the adhesive is heated by laser heating.
 16. A method as claimed in claim 15, wherein the composition includes an infrared absorbent material.
 17. A method as claimed in any of claims 11 to 15, wherein the adhesive is heated by induction or dielectric heating.
 18. A method as claimed in claim 17, wherein the adhesive includes metal particles.
 19. A method as claimed in any of claims 11 to 17, wherein one of both layers comprises polypropylene. 